1. Technical Field
The present disclosure relates generally to the logging of subsurface formations surrounding a wellbore using a downhole logging tool, and particularly to using the logs to estimate the capture cross-section at various depths of investigation, and using those estimates to infer one or more formation properties.
2. Background Art
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other tool types are also used.
Early logging tools were run into a wellbore on a wireline cable, after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, the need for information while drilling the borehole gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, and NMR distributions (e.g., T1 and T2). MWD and LWD tools often have components common to wireline tools (e.g., transmitting and receiving antennas), but MWD and LWD tools must be constructed to not only endure but to operate in the harsh environment of drilling.
It is known how to combine capture cross-section (Σ or Sigma) and phase shift resistivity to determine water saturation (Sw) and water salinity. However, Σ is a shallow measurement (˜9 inches) compared to resistivity (˜16 inches). A logging tool such as Schlumberger's ECOSCOPE tool is capable of measuring capture cross section for multiple depths of investigation ranging from about 6 inches to 12 inches into the formation. There is, therefore, a need to estimate Σ beyond its deepest DOI, especially when multi-DOI Sigma measurements indicate the presence of invasion. Such an estimate would assist in reducing formation parameter estimate errors due to invasion and/or making the Sigma measurement more compatible with the phase shift resistivity measurement used to determine water saturation and salinity.